Permanent magnet rotors for ECM motors typically comprise a solid or laminated iron core, surrounded by a plurality of magnetic elements. The materials typically used to form the magnetic elements include barium or strontium ferrite (traditional ferrite magnets) and samarium cobalt (rare-earth magnets). The former are often referred to as ceramic magnets and are generally manufactured by pressing a granular or powdered ferrite material into a mold of desired shape, and "firing" or heating the molded element in a kiln until cured. Permanent magnet rotors of this type are used primarily in electric motors and in conjunction with control circuitry and other associated apparatus of the type as disclosed, for example, in the following U.S. patents (as well as the applications and patents related thereto), all of which are assigned to the assignee of the present application and incorporated herein by reference: U.S. Pat. No. 4,456,845 (Cunningham); U.S. Pat. No. 4,466,165 (Otto); U.S. Pat. No. 4,476,736 (Hershberger) and related divisional applications listed therein; U.S. Pat. No. 4,654,566 (Erdman) and related continuation and continuation-in-part applications listed therein; and U.S. Pat. No. 4,686,436 (Archer). Rotors produced in accordance with the present invention can be used to particular advantage in hermetically sealed refrigeration compressor applications where exposure to refrigeration and/or lubricating fluids is likely to occur, and where relatively high efficiency ratings may be required.
Due to the relatively high mass of the materials used to form the permanent magnetic elements disclosed in the aforelisted patents, and the relatively high (1,000-16,000 RPM) rotor speeds developed in the associated motors, retention of the magnetic elements in position adjacent the rotor core is a serious concern. A number of methods and techniques for retaining magnetic elements on the rotor core have previously been used or considered. One such technique involves positioning the magnetic elements around the outer diameter of the core, and applying a wrap of Kevlar or fiberglass to hold the magnets in place. The Kevlar or fiberglass used is typically a fine stranded material which is pulled through epoxy prior to wrapping. An adhesive filler may be applied to the wrapped rotor to fill voids and provide a more rigid construction. A drawback in this technique, however, is that the ends of the stranded material are difficult to attach to the rotor core, and the nature of the materials involved may create problems in the working environment. Additionally, use of this technique is relatively expensive and time consuming, and uniformity and tolerances of the finished product can be difficult to control.
Another technique for retaining the magnets on the core involves wrapping a relatively fine wire, under tension, around the magnetic elements, followed by application of an adhesive or epoxy overcoat to the assembly to protect the metal wire. As with the fiberglass wrap technique discussed above, proper attachment of the wire ends to the core is difficult and relatively expensive to achieve. This technique is also time consuming and labor intensive, and involves a number of distinct operations which may be difficult and costly to automate for the production-setting.
Another technique for retaining magnets around a rotor core involves the use of a cylindrically shaped shell or "can" which is disposed around the outer peripheral surface of the magnet/core sub-assembly to hold the magnets in position. The outer shell is typically formed of a non-magnetic steel such as INCO-718 (Inconel) or beryllium-copper. At least three techniques for assembling a permanent magnet rotor which uses such a retaining shell have been previously developed. One of these techniques is understood to have originated with Hitachi, Ltd. of Japan and is believed to involve placing a core and magnetic elements in a shell, die casting molten aluminum into the shell to lock the magnetic elements and core in place, and forming end rings at the ends of the rotor so as to completely cover the magnetic elements. The shells of these rotors appear to be formed from a 300 series stainless-steel having a relatively thick-walled construction which is machined after die-casting to a final wall thickness varying between 0.015" and 0.025" depending on point of measurement.
Another technique for assembling an outer retaining shell onto a core/magnet sub-assembly is described in U.S. Pat. No. 4,549,341 to Kasabian. This patent discloses a rotor formed from a shaft (10) turned to provide a central area (20) having a larger outer diameter than axially adjacent end areas (15 and 24). A plurality of flat faces (30) are formed on the larger diameter portion and each flat has a steel block (50) mounted thereon conforming in shape to a permanent magnet to be ultimately carried on the shaft. A layer of non-magnetic material (60) (e.g., aluminum) is cast around the larger diameter portion of the shaft and attached blocks, and machined to a diameter slightly larger than the desired finished diameter. The blocks are then removed and replaced with permanent magnets (80) which are typically rare-earth permanent magnets formed of samarium cobalt or Al-nickel and are retained in the apertures by magnetic attraction to the shaft or, alternatively, by an adhesive material. After the magnets are installed, the magnets and adjacent casting surfaces are machined to the desired finished diameter, and an outer non-magnetic steel shell, such as INCO-718, is installed over the magnets by heat shrinking to provide an interference fit between the shell and the underlying casting and permanent magnets. Other patents which disclose the use of a heat-shrink technique for installing a retaining shell over a rotor core and surrounding magnets include U.S. Pat. Nos. 3,531,670 and 3,909,647 which are assigned to The Bendix Corporation, and U.S. Pat. Nos. 4,242,610, 4,332,079, 4,339,874, and 4,445,062 and a number of related patents assigned to the Garrett Corporation.
U.S. Pat. No. 4,617,726 (Denk) discloses an alternative technique for installing an outer shell (110) over a rotor sub-assembly (50). This technique utilizes a tubular housing (120) in which the outer shell (110) is supported and subjected to hydraulic pressure so as to cause the tubular shell to expand radially outwardly. The rotor sub-assembly (50) is forced axially into the expanded shell by a ram to effect a uniform fit between the rotor sub-assembly and the surrounding shell.
Each of the aforedescribed techniques for installing a retaining shell over a core and surrounding magnetic elements has limitations and disadvantages. One disadvantage is the requirement for separate casting and machining operations which are costly, time consuming, and potentially injurious to the magnetic elements. The die casting technique initially requires a shell having a relatively large wall thickness due, at least in part, to conditions attendant to die-casting operations. A machining operation is generally required to reduce the wall thickness of the shell in order to avoid relatively large losses in efficiency.
The described heat-shrink technique is also potentially injurious to the magnetic elements since the heat to which the magnetic elements are exposed can cause the magnetic materials to crack and chip. Additionally, the amount of expansion which can be achieved by heating the shell is limited. For example, the diameter of a shell for a three-inch diameter rotor will increase approximately 0,017" when heated. Accordingly, the machining step required by Kasabian prior to heat-shrink installation of his retaining sleeve must be precise to ensure a high interference fit after cooling of the sleeve. Machining operations of the type described by Kasabian are generally not practical when ferrite or other ceramic magnetic elements are used because such materials are very expensive to machine and cut. Moreover, the materials and techniques conventionally used in the manufacture of ceramic magnets lead to relatively wide variations in dimensional tolerances. For example, ferrite magnetic elements of the size which might typically be used with a three-inch diameter rotor may vary by 0,020" in thickness. The overall diameter of the core and surrounding magnetic elements may vary by up to 0,040". Thus, ferrite magnetic element cannot routinely be used in heat-shrink assembly techniques due to the inherent dimensional limitations. Although the hydraulic expansion technique disclosed in U.S. Pat. No. 4,617,726 avoids possible damage to the magnets inherent in the heat-shrink method, this technique does not address the problems associated with dimensional variations in the magnetic elements and core.